Under what account are you trying to push out the Symantec Mgmt Agent install package to your clients?
If you're leaving it the default, the application identity account, then does that account have sufficient privileges to install software on the clients?
If not, you can specify a different account to install the Agent. Under "Rollout Agents to Computers" section, you'll see a "Settings" button. Click that and near the bottom of the window that opens you'll see where you can provide alternate credentials.
IMHO, resolving the app identity account priv's would be your best bet, that way you can use "Scheduled Push to Computers."